The prior art is described with reference to FIGS. 5A and 5B. This is cited from an article released in SSDMS7 (Extended Abstract of Conference on Solid State Device and Materials, p. 319, Tokyo, 1987, Japan Society of Applied Physics). In the prior art, gas supplied to a plasma source is B2H6 diluted to 0.05% by He.
As shown in FIG. 5A, silicon wafer 20 is loaded into reactor chamber 10. After reducing the pressure to 5×10−7 torr (6.7×10−5 Pa), B2H6 gas 30 diluted to 0.05% by He is introduced to attain 5×10−4 torr (0.067 Pa). Under these conditions, power is applied to ECR plasma source 40 to induce high-frequency waves and generate plasma 45. RF power supply 60 then feeds high-frequency waves to wafer susceptor 50 on which wafer 20 is placed. Thus, generates a certain voltage, approximately 700 V according to the cited article, between generated plasma 45 and silicon wafer 20. As a result, positive ion 70 in plasma which contains positive ion 70 and electron 80 is attracted and doped near the surface of wafer 20. In the cited example, the B atom in B2H6 becomes a source of positive charge (a positive hole) in the silicon and the B atom is called a dopant.
In the cited example, the purpose of diluting B2H6 by He is an attempt to increase the safety of B2H6, which is extremely toxic. However, as the partial pressure of B2H6 falls, the density of ions containing B in plasma also falls, resulting in a significant drop in doping efficiency.
Helium used for dilution has small atomic radius, and is employed for dilution because it diffuses outwardly and is removed easily by thermal treatment even after doping He to silicon. However, He's high ionization energy makes it unsuitable for generating and maintaining plasma in some cases.
Moreover, the use of He hinders the controllability of the dopant implantation amount.
Accordingly, such a plasma doping method is needed as: securing higher safety of handling B2H6, which is extremely hazardous to the human body due to its high toxicity, by dilution of B2H6 as far as possible; preventing reduction of doping efficiency; generating and maintaining plasma stably; and facilitating the control of dopant implantation amount.